Patent FR-2,062,138 provides a solution to the removal of COS from natural gas. The method consists in promoting the absorption of COS by an aqueous alkanolamine solution by means of an absorption zone maintained at a temperature ranging between 60° C. and 90° C., preferably between 70° C. and 80° C. The aqueous alkanolamine solution then allows to collect the CO2 and the H2S from the hydrolysis reaction in another zone operating under temperature conditions favoring removal of these two compounds. It is however necessary, according to patent FR-2,062,138, to first remove the CO2 and the H2S present in the gas to be treated so as to promote the hydrolysis phenomenon.
U.S. Pat. No. 6,852,144 also describes a method of removing COS from hydrocarbons. The method uses a water-methyldiethanolamine or water-triethanolamine solvent with a high proportion of a compound belonging to the following group: piperazine and/or methylpiperazine and/or morpholine, allowing to achieve selective absorption of COS in relation to carbon dioxide.
Patent FR-2,888,247 can also be mentioned, which relates to a method of removing the COS contained in a liquid hydrocarbon feed by contacting the liquid hydrocarbon feed with a conventional alkanolamine solution by means of a membrane contactor.
It is however well known that the primary or secondary alkanolamines used in the methods described above promote a strong reaction with COS, with a quite significant risk of degradation in the presence of COS. Authors Kohl and Nielsen [Gas Purification, 5th Edition, Gulf Publishing Company] provide a brief description of the main alkanolamines used for treating natural gas. They show in their book that the reactivity of alkanolamine towards acid gases is correlated with the level of substitution of the nitrogen atom: the more the nitrogen atom is substituted, the less the alkanolamine is reactive. Primary alkanolamines are thus more reactive than tertiary alkanolamines. This reactivity can be related to the stability of the reaction products formed. Thus, the products resulting from the reaction of an acid gas with a primary alkanolamine are more stable than those resulting from a reaction with a tertiary alkanolamine. The regeneration of primary alkanolamines is therefore generally more difficult. Furthermore, it has been observed that primary alkanolamines involve a high degradation risk in the presence of COS. Direct reaction of COS with monoethanolamine (MEA), a primary alkanolamine, thus leads to non-regeneratable stable products. Diethanolurea, 2-oxazolidone and (2-hydroxyethyl)imidazolidone are the main products of the degradation of MEA in the presence of COS. This degradation process is closely linked with the operating conditions of the solvent regeneration stage. MEA and most primary alkanaolamines are therefore not considered as regeneratable solvents for COS removal.
Diethanolamine (DEA), a secondary alkanolamine, is also widely used for the removal of acid products from a gas or liquid effluent. Its basicity allows total deacidizing and deep removal of COS. According to Dawodu (1991), DEA degrades in the presence of COS and gives water-soluble degradation products, as well as a sulfur-rich insoluble solid. Despite this drawback, DEA remains one of the most widely used alkanolamines for deacidizing a gaseous effluent.
Methyldiethanolamine (MDEA) is the most widely used tertiary alkanolamine, generally for selective removal of H2S in relation to CO2. The COS removal performances of MDEA are poor and it is necessary to add a kinetic activator, generally a primary or secondary alkanolamine, to optimize the efficiency of the solvent used in the method.
The object of the present invention thus is to overcome one or more of the drawbacks of the prior art by providing a method allowing removal of acid gases, such as CO2, H2S and COS, from natural gas using a primary alkanolamine of higher chemical stability towards COS than those of the prior art.